Missing self in image_slices_viewer

Little bug that shows up only when using the class in a different context:

https://github.com/matplotlib/matplotlib/blob/4aa27080672d178c03c0c37a65751b3a04c840bd/examples/event_handling/image_slices_viewer.py#L35

should be `self.ax.set_ylabel(...`. It feels silly to submit a PR for 5 characters, but can do it if that's the procedure.
